Qualitative analysis Flashcards
How do you carry out a carbonate test?
Carbonates react with acids to form carbon dioxide gas:
Na2CO3 (aq) + 2HNO3 (aq) -> 2NaNO3 (aq) + CO2 (g) + H2O (l)
This reactionf forms the basis for a test for the carbonate ion, CO3 2-
1) In a test tube, add dilute nitric acid to the solid or solution to be tested
2) If you see bubbles, the unknown compound could be a carbonate
3) To prove that the gas is carbon dioxide, bubble the gas through lime water - a saturated aqueous solution of calcium hydroxide Ca(OH)2. Carbon dioxide reacts to form a fine white precipitate of calcium carbonate, which turns the lime water cloudy (milky)
CO2 (g) + Ca(OH)2 (aq) -> CaCO3 (s) + H2O (l)
How do you carry out a sulfate test?
The formation of a white precipitate of barium sulfate is the basis for the sulfate test, in which aqueous barium ions are added to a solution of an unknown compound. The ionic equation is shown below:
Ba 2+ (aq) + SO4 2- (aq) -> BaSO4 (s)
Usually the Ba 2+ (aq) ions are added as aqueous barium chloride or barium nitrate. If you intend to carry out a halide test afterwards, use barium nitrate - with barium chloride, chloride ions are introduced into the solution
How do you carry out a halide test?
Aqueous silver ions react with aqueous halide ions to form precipitates of silver halides, as shown by the general equation below. X- (aq) represnts an aqueous solution of any halide
Ag+ (aq) + X - (aq) -> AgX (s)
This reaction forms the basis for a halide test:
1) Add aqueous silver nitrate, AgNO3 to an aqueous solution of a halide
2) The silver halide precipitates are different colours - the silver chloride is white, silver bromide is cream and silver iodide is yellow
3) Add aqueous ammonia to test the solubility of the precipitate. This stage is very useful because the 3 precipitate colours can be difficult to tell apart
AgCl (s)
- White precipitate
- Soluble in dilute NH3 (aq)
AgBr (s)
- Cream precipitate
- Soluble in concentrated NH3 (aq)
AgI (s)
- Yellow precipitate
- Insoluble in concentrated NH3 (aq)
What is the sequence of the qualitative analysis tests?
1) Carbonate, CO3 2-
2) Sulfate, SO4 2-
3) Halides, Cl - , Br - and I -
Why does the carbonate test go first in the sequence?
In this test you add a dilute acid and are looking for effervescence from carbon dioxide gas. Neither sulfate nor halide ions produce bubbles with dilute acid. The carbonate test can be carried out without the possibility of an incorrect conclusion. If the test produces no bubbles, then no carbonate is present and you can proceed to the next test
Why does the sulfate test go second in the sequence?
In the test you add a solution containing Ba 2+ (aq) ions and are looking for a white precipitate of BaSO 4 (s). Barium carbonate, BaCO3, is white and insoluble in water. So if you carry out a sulfate test on a carbonate, you will get a white precipitate too. Therefore it is important to carry out the carbonate test first and only proceed to the sulfate test if you know there is no carbonate present
Why is the halide test go third in the sequence?
In the test you add a solution of Ag + (aq) ions, as AgNO3 (aq) and are looking for a precipitate. Silver carbonate, Ag2CO3, and silver sulfate, Ag2SO4 are both insoluble in water and will form as precipitates in this test. It is therefore important to carry out the halide test last, after carrying out carbonate and sulfate tests to rule out those possibilities
How do you analyse a mixture of solutions using the same solution?
1) CARBONATE
- If you see bubbles, continue adding dilute nitric acid until the bubbling stops
- All carbonate ions will have been removed and there will be none left to react in the next tests
- It is important to use HNO3 for this test as sulfuric acid will react in the sulfate test and hydrochloric acid will react in the halide test
2) SULFATE
- To the solution left from the carbonate test, add an excess of Ba(NO3)2 (aq). Any sulfate ions present will precipitate out as barium sulfate
- Filter the solution to remove the barium sulfate
- It is important not to use BaCl2 because the chloride ions will show up in the halide test
3) HALIDE
Carry out as normal
How do you test for the ammonium ion?
When heated together, aqueous ammonia ions and aqueous hydroxide ions react to form ammonia gas, NH3
NH4 +(aq) + OH - (aq) -> NH3 (g) + H2O (l)
This reaction forms the basis for a test of the ammonium ion
1) Aqueous sodium hydroxide, NaOH is added to a solution of an ammonium gas
2) Ammonia gas is produced. You are unlikely to see gas bubbles as ammonia is very soluble in water
3) The mixture is warmed and ammonia gas is released
4) You may be able to smell the ammonia, but it is easy to test the gas with moist pH indicator paper. Ammonia is alkaline and its presence will turn the paper blue
